1. Field of the invention
The invention relates to the field of liquid level displays. It proceeds from a method for liquid level measurement and a capacitive liquid level sensor according to the preambles of claims 1 and 4.
2. Discussion of Background
There are known in the prior art a multiplicity of devices for determining the liquid level in a container which are based on very different physical measurement principles. These comprise electrical (capacitive or resistive) and optical methods, radar reflection methods, ultrasonic echo time methods and gamma-ray absorption methods.
Use is made in offshore petroleum recovery of so-called separation tanks, in which the different phases (sand, water, oil and gas) occurring during drilling or recovery are separated on the basis of their density differences and led off in separate conduit systems. It is very important in this regard to know the height of the separating layer between the water and oil, in order to be able to open and close the outlet valves for the two media on the tank in a controlled fashion. Reliable liquid level measuring instruments are required for this purpose. If such a liquid level measuring instrument does not function or functions incorrectly, it is possible, for example, for oil to reach the water outlet and cause considerable environmental damage and costs.
Recently, high-pressure separation tanks have been developed which are suitable for operation on the sea bed a few 100 m below the surface of the sea. The oil, recovered and already separated, can then be pumped to the surface of the sea with a much lesser expenditure of energy. Such separator tanks are exposed to very high pressures of 60 bars-180 bars, and to high temperatures of 50-120.degree. C. The liquid level measuring system must function all year round without maintenance and reliably under these difficult conditions, since an operational failure and premature replacement would entail high costs.
In an earlier German patent application (reference number 197 13 267.7) which is not a prior publication, there is proposed as a solution to this problem a capacitive measuring probe which makes use, in particular, of the large jump between the dielectric constants at the interface between oil and water. The rod-shaped, inherently closed probe carries a plurality of annular or toroidal electrodes, partially segmented in the longitudinal direction, along the axis. The stray capacitance between neighboring electrodes varies with the dielectric constants of the ambient medium. The capacitance signals of the electrode pairs can be used individually for a digital liquid level display, or averaged for an analog liquid level display. In both cases, the spatio resolution is limited by the periodicity of the electrode arrangement. Liquid levels situated therebetween cannot be measured. Another disadvantage is that interfaces between media of the same dielectric constant cannot be detected. As an alternative, or in addition, it is proposed for media of different conductivity to measure the ohmic discharging currents of electrode pair capacitances in order to locate the interface. This requires a measuring frequency to be selected so low that the ohmic current dominates over the capacitive one in both media. However, no account is taken of effects of the conductivity of the media on the geometry and size of the effectively active electrode pair capacitances.
Pollution of the measuring probe by the media to be measured constitutes a considerable problem for the measuring accuracy and reliability of such liquid level sensors. For example, a conductive water film can complicate or preclude the detection of an oil medium, and a non-conducting oil coating can complicate or preclude the detection of a water medium. In the abovenamed patent application, it is proposed to design the probe specifically for close-range and long-range measurements: the stray capacitances are a measure for the close range of the measuring probe when the electrodes are of large area and/or widely spaced, and are a measure for the close range when the electrodes are of small area and/or closely spaced. However, it is disadvantageous that the optimum electrode geometry is specified only with regard to the range of the stray fields in the dielectric media and, moreover, only qualitatively. Finally, the long range can also be effectively detected by capacitance measurements between the probe and an additional counter electrode, for example the container wall. Furthermore, the close range can be measured in conductive media using the skin effect. However, very high frequencies are required for penetration depths in the mm region -10 GHz and above, for example, in the case of water.
DE 28 19 731 discloses a capacitive sensor which serves to monitor the limiting value of an adhesive, conductive medium. The capacitance measurement is performed between a rod-shaped probe and the container wall. Conductive deposits on the probe and the wall can cause fault currents which are detected by a shielding electrode along the probe axis. The fault currents can also be kept low by measuring frequencies which are as high as possible. A disadvantage of this arrangement consists in that the liquid level height cannot be measured continuously.
DE 38 12 687 A1 discloses a capacitive sensor whilst simultaneously determining the level and the water content of an at least weakly conductive liquid. For this purpose, two coaxially arranged electrodes are dipped into the medium and the complex impedance is measured at at least two frequencies. The capacitive reactance is a measure of the liquid level, and the ohmic effective resistance is a measure of the conductivity of the medium.
None of the said sensors takes account of the fact that the measurement of capacitance in conductive media is influenced by ionic charge transport, something which gives rise to new possibilities for eliminating the disturbing influences of insulating and conductive pollutant films. Moreover, new conditions can be specified for optimizing the electrode geometry.